This invention relates to loudspeakers for providing sound from electrical signals which may be analogue or digital in nature.
Conventional analogue loudspeakers rely for their operation on the motion of a diaphragm which is driven by some type of electromechanical motor, moving-coil being the most common variety, though electrostatic, piezoelectric and ionisation devices have all been tried and used. The analogue loudspeaker as a whole attempts to reproduce the desired sound by moving all or part of the diaphragm closely in synchronism with a smoothly varying analogue electrical signal which is usually interpreted as representing the instantaneous sound pressure that a listener to the loudspeaker device should hear. The inherent limitations of such analogue loudspeakers are in part related to stiffness of the diaphragm used, mass of the diaphragm, the linearity and efficiency of and power available from electromechanical motors with adequate bandwidth, and limitations on throw of the diaphragm. These and other factors combine to cause the analogue loudspeaker to operate with low efficiency and relatively high distortion levels.
With the current prevalence of high quality digital audio material available, frequently in 16-bit binary format with an inherent distortion level of close to 0.002%, it is clear that current analogue hi-fidelity loudspeaker systems operating close to the 1% distortion level (500 times worse) are now the limiting factor in audio quality when listening to reproduced sound. Recent trends in electronic equipment have also been to mininise power consumption, not only to reduce power wastage, but also to reduce equipment operating temperatures thus allowing mniniaturisation and high reliability, as well as portability, and allowing operation from small batteries. Again, the linear analogue power amplifier/loudspeaker combination operating at the 0.3% to 1% electro-acoustic efficiency level is out of step with these trends. Lastly, even though digital audio source material is now commonplace and becoming increasingly so with the advent of digital radio and television, all conventional hi-fidelity systems for the reproduction of digital source material need to contain a digital-to-analogue converter (DAC) at some point in the system, to produce analogue signals for application to the analogue loudspeaker. The DACs themselves produce further noise and distortion that adds to that already present in the system, and also add extra cost.
Attempts have been made previously to develop a digital loudspeaker design that overcomes some or all of the limitations of analogue loudspeakers mentioned above. These fall into several categories: Pseudo-digital loudspeakers comprising a digital signal processor driving a standard analogue speaker, Moving Coil Digital Loudspeakers with tapped xe2x80x9cvoice-coilsxe2x80x9d; Piezoelectric and Electrostatic drivers, where the area of the diaphragm is divided into separate regions with binary-related surface areas; and Pulse-width-modulation amplification which is really a digital amplifier technology. Most previous attempts at building a digital loudspeaker system have assumed that binary digital code was the digital signal medium, not only at the input of the device but also right through to the output transducers. This causes serious technical problems in practice.
In a signed n-bit system, the transducer used for the least significant bit (LSB) of the output operates at a power level 2nxe2x88x922 times less than the most significant (non-sign) bit (MSB). Because of the necessarily mechanical nature of sound-producing devices, this wide dynamic range imposes serious design constraints on the types of devices used for LSB and MSB transducers. and thus makes matching of the devices very difficult. In a binary-weighted transducer (or transducer-array) system, there are serious transient problems caused at points where the code changes from a value with many consecutive low order zeroes or ones to the next level (up or down) where there are many consecutive low order ones or zeroes. Multiple acoustic transitions occur at this code point change which will inevitably produce considerable sound energy, even though the code change represents only a least significant bit change in signal amplitude which would be preferably nearly inaudible.
In addition to the switching transient problem outlined there is also a level error associated with such zeroes-to-ones and ones-to-zeroes code changes. This is because in a real system the transducers cannot easily be matched precisely enough that the each transducer is precisely one least significant bit greater in effective power or amplitude than the sum of all the lesser-bit transducers acting in concert.
It is not unknown for loudspeakers to have arrays of many transducers which produce pressure pulses, individually and independently fed. But in the past this has been done with binary digital signals, e.g. Stinger U.S. Pat. No. 4,515,997, which fails practically for the reasons given above. No one has done it with unary digital signals because their advantages in these respects were not foreseen. Voltage-sampled signals, not discrete-time-sampled signals, have been used to determine what instantaneous number of identical transducers, arranged in a one or two dimensional array, should be switched on as a series of voltage thresholds has been reached, e.g. Nubert, DE Patent 4343807 A1, but this does not disclose unary encoding of binary (or other) digital codes, nor does it usefully convert the voltage levels that cause triggering of the various transducers into appropriate sound pressure levels, instead triggering near-instantaneous 3-dimensional-volume changes in the transducers, or constant strokes, thus creating a series of positive and negative pressure impulses at the trigger edges rather than continuous pulses of an appropriate polarity, the end result being the production of no useful sound power. Nubert, having described the operation of his invention entirely in terms of events occurring at certain voltage levels, does also suggest that analogue signals could be converted to digital form with an ADC; however, this in itself does not necessarily suggest regular time-sampled digital signals as, e.g., flash-ADCs are capable of digitizing without regular (or in some cases, any) clock signals. There is no clear statement at all in Neuberg""s disclosure as to how to achieve control of individual transducers in the case w here a digital input signal is present and in particular, no clear indication of what and how to encode such digital input signals into any required form. Neuberg also discloses the idea of using constant stroke transducers, but does not disclose constant pressure pulse transducers.
Another problem not adequately addressed by existing digital loudspeaker designs is that of transducer dynamics and appropriate drive waveforms for producing the desired acoustic sound output waveform.
Binary to unary ends are known in the prior art U.S. Pat. No. 5,313,300 (Rabile) describes a technique of synthesising such encoders for video DACs, for wide binary words from unary sub-encoders each capable of encoding less-wide binary words. The technique described uses xe2x80x98tiersxe2x80x99 of small unary encoders interconnected in something like a tree-structure, with additional gating blocks to perform the final conversion to unary. A problem exists with extending this design to larger input binary bit-widths in that the interconnection system between all the unary sub-encoders becomes complex and is not amenable to a bus-structured approach (because of its tree-like tiered or cascade nature). In addition, U.S. Pat. No. 5,313,300""s design presupposes the existence of the unary subecoder blocks but nowhere describes the operation (nor truth table) of such sub-encoders and their exact nature is therefore unclear, although a truth table for an entire encoder is disclosed.
Digital Pulse Width Modulation, PWM, generators are known e.g. Kirn U.S. Pat. No. 4,773,096, which use a clock oscillator to drive a digital counter whose outputs are connected to one input of a digital magnitude comparator, to convert a series of digital input words into a series of PWM waveforms whose successive mean values approximate the values of the series of digital input words. However, such devices do not convert single digital input pulses into PWM ramp signals as per the present disclosure.
Rogers U.S. Pat. No. 5,287,531 discloses a means of reading data from a series of plug-in cards in a computer bus, by daisy chaining a series of serial input/output shift registers, SISOSR, one per card, and clocking their contents out in sequence into the controlling microprocessor. However, the amount of data retrievable from each card with this scheme is completely limited by the size of SISOSR installed on each card, and no capability to write information uniquely to each card, to store information or to control the cards, is disclosed therein. Roger""s device apparently detects the number of devices on the bus by reading dummy data from the SISOSR which is injected at the end of the bus furthest from the controlling microprocessor and which data is arranged to be different from the data produced by any valid card that might be plugged into the box, and is then interpreted as such by the processor; it does not produce a distinct logic pulse on a dedicated line to indicate that the last device on the bus has been detected.
A unary digit can take on any of the two values 0, 1 or alternatively can be defined to take on only the single value 1, and its absence is then used to represent 0 somewhat as in Roman Numeral notation. Unary integer positional notation is similar to binary or decimal positional notation except that integer powers of one are used instead of powers of 2 or 10. As all positive integer powers of 1 are equal to 1, with a unary representation all digits have equal weight, and that weight is unity and the position of a unary digit in a unary positional notation number is irrelevant, only its value of 1 or 0, or alternatively, its presence or absence, having any significance. Thus e.g. the 4th unary digit from the right in a unary positional integer represents a factor of 1 or 0 times 13=1, and the first unary digit at the right represents a factor of 1 or 0 times 10=1. So, e.g.
110101=1xc3x9714+1xc3x9713+0xc3x9712+1xc3x9711+0xc3x9710=110+110+0+110+0=310
which is just the number of 1-digits in the number. Thus digit position becomes irrelevant in unary numbers. It is for this reason that the 0 is not needed since its use as place-keeper in positional notations is irrelevant in the unary case. Thus we may just as precisely write the number 110101 as 1111 with both representations having the decimal value 310. It is important to realize that in unary representation it is the number of one-digits that matters, not the position of the one-digits.
Terminology
There is no special name for decimal digits. It is conventional to abbreviate the phrase binary digit to bit. Similarly, it is conventional to abbreviate unary digit to unit. However, as the word unit is easily confused in this unfamiliar role with its more conventional meaning, we use the phrase unary digit.
A loudspeaker comprising a number of substantially identical transducers each arranged to convert an electrical loudspeaker-input-signal into an acoustic output, wherein each transducer is drivable independently of all of the others by discrete time-sampled digital signals representative of the sound to be produced by the loudspeaker, and wherein each of the transducers is such as to produce pressure pulses when driven by an electrical signal, so that the cumulative effect of the transducers is to produce an output sound representative of the input signal, characterized in that the signals by which each transducer is driveable are unary digital signals at a predetermined sampling rate, and the loudspeaker includes encoder means for converting a non-unary digital input signal at a predetermined sampling rate into a plurality of unary digital signals at said predetermined sampling rate and each transducer produces an approximately constant pressure pulse for the duration of a unary digital drive signal pulse.
The invention may additionally comprise pulse shaper means for converting the unary digital signals into a variety of square and non-square profile pulse signals appropriate to the type of transducers used. The transducers in a preferred embodiment are bipolar, being capable of producing positive and negative pressure changes dependent on the polarity significance of the unary signal applied.
In one preferred embodiment the transducers are arranged in a two-dimensional array. The shape of each transducer may be such that they tessellate in two dimensions, e.g. being triangular, square, rectangular or hexagonal. Gaps between the transducers may optionally be provided in this case. Alternatively the shape of each transducer may be such that the transducers do not tessellate, e.g. being circular or oval, gaps being provided between adjacent transducers. The presence of these gaps in a first array of transducers can be exploited by providing a further array of transducers behind the first array, each transducer in the second array being located behind a corresponding gap in the first array, making the arrangement three-dimensional. This process may be repeated to provide a composite transducer array with any number of layers.
As the transducer array is distributed in space, either in two or three dimensions, a listener will be distanced from the transducers by varying amounts, dependent on the position of any particular transducer in the array, with the effect that acoustic pulses emitted simultaneously by the transducers will arrive at the position of the listener at different times. This effect can be corrected by introducing a delay means for differentially delaying the input signals to the transducers in dependence upon their distance from the listener such that the acoustic pulses from all the transducers resulting from a single input signal change to the loudspeaker arrive at the position of the listener simultaneously. Further, the delay means may be adjustable to vary the delays applied dependent upon a chosen, and possibly varying, position of the listener.
The individual transducers in the invention as described above may alternatively be comprised of pairs, triplets, quadruplets or in general N-tuplets (N greater than =1) of independent transducer elements arranged in a two or three dimensional array, with each of the transducer elements comprising an N-tuplet being driven by the same unary signal and being positioned in the array such that the position of the centre of gravity of each N-tuplet taken as a whole lies as close as possible to the vertical or horizontal centre lines of the array, or both, and in the case of a three dimensional array as close as possible also to the front-to-back centre plane of the array, in such a way as to localise the perceived sound from the loudspeaker to as small an area near the centre of the array as possible. This technique allows the construction of a large digital loudspeaker comprised of arrays of transducers where the spatial extent of the array is comparable to the distance of the listener from the loudspeaker, whilst still producing the illusion of being a spatially small sound source with a definite location, close to the centre of the array.
The output sound produced by the array of transducers is the additive effect of the individual sounds produced by the individual transducers. No individual transducer reproduces the desired sound. In the case where the level of drive to each transducer is fixed, quieter sounds will be reproduced as a result of activation of a smaller number of transducers than louder sounds. The effect of encoding the input signal into a unary format is that an M out of N encoding is produced, where N+1 is the number of distinct levels (including zero) representable by the input signal and N is the maximum number of transducers required, and M is the instantaneous input signal level and is also the number of transducers activated by that input signal level.
Because in a unary encoded digital loudspeaker system all transducers have the same, unit, output power level or xe2x80x98weightxe2x80x99, from the point of view of resultant sound output pressure, it does not matter which particular M transducers are turned on out of the total set of N transducers, to produce an output pressure level of M/N of the maximum available. Thus a degree of flexibility is available in choosing subsets of transducers from the whole array which may be used to enhance performance in a variety of ways.
Preferably, the transducers associated with similar input signal levels are physically adjacent in the array so that a reasonably localised sound source results, particularly at low amplitudes of reproduced sound.
In order to reduce the acoustic emission of the loudspeaker in the frequency region above the limit of human hearing (ultrasonic emission), e.g. at frequencies greater than about 20 KHz, an acoustic low pass filter may be added between the output transducer array and the listening space. This may be implemented by the positioning of an appropriate quantity of material with high sound absorption in the region above 20 KHz and low sound absorption below that frequency, between the acoustic output transducers and the listening space.
Ultrasonic emission from the loudspeaker may also be reduced by increasing the digital sampling rate as much as possible. Standard digital audio material such as that available from compact discs and other common sources has a sampling rate in the 40 KHz to 50 KHz region. When reproducing a 20 KHz audio input signal with such a sampling rate, only two or three samples will occur within each cycle of the input signal. If the same sampling rate is carried through all the way to the acoustic output transducers, significant acoustic energy will be emitted below 100 KHz and smaller amounts at higher frequencies. Raising the sampling rate to e.g. 100 KHz places the lowest frequency strong supersonic emission at a significantly higher frequency and reduces its amplitude proportionately. The invention optionally includes digital interpolating means to raise the sampling rate of the input signal to the loudspeaker. In this invention the interpolation process is used to ease the acoustic filtering requirements after digital to analogue sound conversion, not the electrical filtering.
The encoder means may have a plurality of parallel outputs corresponding to the number of unary signals and the number of transducers. An alternative arrangement is for the unary signals to be compressed in time and for the encoder to have fewer outputs, in the limit a single output, means then being provided to reconstitute the unary signals as a parallel stream for application to the transducers.
Preferably, the loudspeaker assembly according to the invention includes transducer drivers connected between the encoder means and the transducers, the transducer drivers converting the unary output signals from the encoder means to appropriate current and voltage levels to drive the transducers.
Preferably the loudspeaker assembly incorporates additional pulse shaping means controlling the shape of the driving waveform to the transducers, the pulse shaping means being able to provide driving pulses deviating from the nominal square shape of a standard digital pulse. Where the transducers are such that over the range of speeds of operation appropriate to use as elements of a digital loudspeaker their dynamics are dominated by resistive or viscous drag forces, then a square drive pulse will provide approximately constant velocity operation while the pulse is on and thus an approximately square wave (constant) pulse output pressure will result. Where the transducer dynamics are such that spring-like restoring forces (compliance) dominate, generally the case with transducers operating below their resonant frequencies and with low damping, then the pulse shaping means may provide linear ramp shaped driving pulses. Where the transducer dynamics are such that inertial forces dominate, generally the case with transducers operating above their resonant frequencies and with low damping, then the pulse shaping means may provide bipolar impulse shaped driving pulses comprising a short pulse coincident with the leading edge of the input pulse and a second short pulse of reverse polarity coincident with the trailing edge of the input pulse. Where the dynamics of the transducers are composites of these three cases the pulse shaping means may provide a driving pulse waveform such as to produce essentially constant pulse output pressure for the duration of each input pulse. The pulse shaping means may provide any combination of some or all of the pulse shapes cited above and may additionally be combined directly with the transducer driving means into a composite structure. Alternatively, the pulse shaping means may be interposed between the encoder means and the transducer driver means. Another alternative is to interpose the pulse shaping means between the transducer driver means and the transducers.
In order to preserve the generally high power efficiency of digital pulse drive electronics when producing square drive pulses, the pulse shaping in one preferred embodiment of the invention is implemented using pulse width modulation (PWM) techniques wherein the effective shape of the drive pulse to the transducer is the mean-value of a rapidly changing square waveform with many cycles occurring within the duration of one unary input pulse and whose mark-space ratio is varied continuously as required in order to produce the desired effective pulse shape suited to the transducer dynamics.
In a further preferred embodiment the encoding means for producing N unary signals from n input binary (e.g.) bits, where N=2nxe2x88x921 or if one of the n input bits is used as a sign bit then N=2nxe2x88x921, is built in a modular fashion such that a number of identical encoding sub-modules are connected to a data bus carrying the complete input binary (e.g.) data words representing the electrical input signal to be reproduced as sound. The encoding sub-modules, which are each designed to encode P unary digits where P less than N and where usually there would be Q such modules such that Pxc3x97Q=N, are pre-programmed before activation as encoders by sending to them control signals via a control bus and programming data via the data bus or the control bus such that each of the Q sub-modules after programming is responsive to a different group of P input signal levels and encodes just that group of P input signal levels into P unary output signals. The net effect is to encode all N possible input signal levels into Pxc3x97Q=N unary output signals but without the complexity of a brute-force n-bit binary (e.g.) to unary encoder, instead using Q identical modules which are easier to design and mass produce, and also easier to expand to different numbers of input signal bits n. The programing system may be made extremely simple by arranging for each of the Q encoder sub-modules to contain a flip-flop, and arranging for the control-bus connection between the modules to interconnect the Q flip-flops so as to form a serial input shift register. At programming time a single-access pulse AP is introduced to the input of the shift-register so formed after initially clearing it, and which shift register is physically distributed amongst all Q encoder sub-modules, and which is then clocked via a common clock signal present on the control bus, through the shift register one flip-flop at a time. As only one AP is introduced into the input of the flip-flop during programming, only one module can contain the AP after each clock pulse has moved it to the next stage, and therefore if the flip-flop in each module is used to activate that module for programming if and only if it contains the AP, each module may be programmed uniquely in turn by introducing programming information onto the common data bus for example, and issuing a programming pulse onto the control bus common to all modules whereupon only the module containing the AP in its flip-flop will respond to that programming instruction. Thus by shifting the AP through the Q modules one module at a time by means of the clock signal and issuing programming information after each such shift operation, the entire chain of modules may be programmed each with information specific to that module, even though the modules are logically identical and have no hardware unique address as such. This module programming technique is widely applicable to any programmable modular structure connected to a common bus and is not restricted in its use to the digital loudspeaker design presented here.
The encoding means for converting digital input in one form, e.g. binary, to unary digital output, may be simplified where the input form represents a signed quantity, by taking the sign information outside of the encoding scheme and using it together with the encoder outputs to control the transducer drivers or pulse shaping means directly to control the sign of the output signals. In the case of a binary to unary encoder with n input bits, where one of the input bits is a sign-bit, if the other nxe2x88x921 bits are fed to an unsigned nxe2x88x921 bit binary to unary encoder and the 2nxe2x88x921xe2x88x921 unary digital output signals are fed to the transducer drivers together with the input binary sign-bit, a considerable saving in circuitry results with no loss of information.
The effective amplitude of the acoustic output pulses emitted by the unary output transducers may be adjusted whilst still maintaining high efficiency by gating the transducers on and off with a high frequency signal superimposed on top of the drive signals from the unary encoder outputs and any pulse shaping circuitry, in a logical AND manner, where the mark space ratio of the high frequency signal is continuously variable from 0 to 1. This is similar to PWM but is an additional modulation to that already produced by the loudspeaker circuitry. An alternative or possibly additional way of altering the effective amplitude of emitted acoustic output pulse from the transducers is to vary the power supply voltage to the transducer driver circuitry, which again may be done with high efficiency by use of PWM techniques. Both of these techniques allow a volume control function to be incorporated into the loudspeaker whilst maintaining the highest possible signal to noise ratio as the effective attenuation of the volume control occurs right at the output end of the loudspeaker system and thus attenuates any internally generated noise equally with the signal.
The method described in the preceding paragraph may be used to reduce the number of transducers required for a unary digital loudspeaker without reducing the effective resolution of the sound output. This is preferably achieved by the incorporation in the loudspeaker assembly of power control means such as described in the previous paragraph which dynamically vary the output power of each and every transducer in dependence upon the amplitude of the input signal. The power control means may include a digital delay device capable of storing, at full input signal resolution of n bits (e.g. if the input signal is encoded in binary), at least one half cycle of the input signal at its lowest frequency, storage means for storing the maximum amplitude attained by the input signal in the time duration for which the input signal is stored in the delay device, means for selecting the p most significant consecutive input signal bits (p less than =n) containing a 1 and not a 0 in the most significant bit position of that group of p bits and discounting the sign bit, for transfer to the unary encoder, and means for selecting the output power level of the transducers in dependence upon the maximum amplitude attained in the storage means, the selected power level prevailing whilst the stored input signal is read out of the digital delay device. In this manner, a digital loudspeaker capable of encoding  less than =p bits into unary encoded signals driving 2P output transducers is able to produce a dynamic range of n bits (p less than =n) whilst avoiding the extra complexity of providing the additional circuitry and transducers required by an n bit binary to unary encoder and output system.
In order to allow analogue signal sources as well as digital signal sources to be reproduced through the digital loudspeaker that is the subject of this invention, an analogue to digital converter may additionally be incorporated into the loudspeaker assembly to facilitate this function.
To provide both positive and negative pressure changes, separate positive and negative pressure transducers may be provided, or the same transducers may be driven in a bipolar manner. To reproduce silence, all transducers are turned off such that they are stationary. To produce a positive pressure the front face of the transducers is made to move outwards relative to the off-state. To produce a negative pressure the front face of the transducers is made to move inwards relative to the off state. If separate unary digit signals from the output of the binary to unary decoder represent positive and negative pressures, it is possible to either apply these signals to separate positive-pressure and negative-pressure producing transducers, or to drive individual transducers in a push-pull or bipolar manner, with a pair of unary signals driving each one. The latter scheme reduces the number of individual transducers required for a digital loudspeaker of a given resolution by a factor of two. Alternatively, the sign bit of a binary input signal may be omitted from the binary to unary encoder and separately used to control the polarity of pressure pulses from the acoustic transducers driven by the (positive) unary outputs of the encoder. This scheme similarly reduces the number of transducers required for a digital loudspeaker of a given resolution by a factor of two.
A practical digital loudspeaker of this type will require a large number of transducers. E.g. to handle an 8-bit binary input, requires the representation of 256 sound pressure levels. As level 0 requires no pressure, no transducer is required for this level. Therefore, 255 transducers (maximum) are required in this example. If the transducers are driven in a bipolar manner as described above then 128 transducers will suffice. In general, for a system handling n-bit binary input, either 2nxe2x88x921 unipolar or 2nxe2x88x921 bipolar transducers are required. Whilst it is possible to use discrete transducers for this purpose, it is possibly advantageous to use integrated multiple transducers to reduce cost and manufacturing complexity. For example, if electrostatic transducers were to be used, it would be possible to produce a large number of electrodes of equal area, each with a separate connection to separate unary digital signals, on one physical transducing device, thus producing a transducer array. If piezo-electric transducers were to be used, then one piece of piezo-electric material could be divided up into a large number of equal area regions each with its own electrodes for separate connection to distinct unary digital signals, again resulting in a transducer array. Similarly, with an electromagnetic transducer, a set of separately connected wires preferably fabricated by printed circuit technology on a common substrate each producing identical ampere-turn effects within the magnetic field of the device, and individually connected to distinct unary digital signals, would again result in a transducer array. All of these array structures could additionally be operated in a bipolar or push-pull fashion so that each transducer element of the array had separate connections to two distinct unary digital signals or a unary digital signal and a sign control bit for producing positive and negative output pressures. All such array structures have the great advantage of requiring multiple identical elements which assists with matching and simpler manufacture.